1. Technical Field
The invention is related to microstructural devices such as micro-machined silicon transducers of the type employed as accelerometers, seismometers, geophones, magnetometers or pressure gauges and the like.
2. Background Art
Microstructural devices such as micro-machined silicon structures are fabricated using photolithography and processing techniques such as RF plasma etching and the like to form structures having micron or sub-micron feature sizes. Such structures provide extremely sensitive transducers. For example, a micro-machined silicon accelerometer consists of a known proof mass of micro-machined silicon supported on elastically deformable springs of micro-machined silicon and electrostatic platens consisting of thin metal patterns formed on micro-machined silicon surfaces. Measuring acceleration consists of measuring displacement of the proof mass, or the voltage required to maintain the proof mass at a predetermined null position, relative to one of the platens. This may be accomplished in a variety of ways, including capacitive measurements, electron tunneling or optical measurements, as but three of other possible examples.
There are various problems associated with such devices. One example is that the moving parts (such as micro-machined proof mass and spring structures in a sensor for measuring extremely small accelerations in the micro-G and sub-micro-G ranges) are extremely fragile. Forces generated by even the most careful handling during shipping or set-up will destroy the device if steps are not taken to counter these forces.
Another problem is that the calibration of such devices is difficult, particularly on-site calibrations during use. A related problem is that the characteristics or responsiveness of such a device is prone to drift over time or temperature, requiring frequent calibration during use to maintain the extremely fine accuracy of such a device.
An additional problem is that characterizing a micro-machined transducer is difficult because the device must be moved or accelerated while its response is being recorded or observed. For example, it may be desired to measure the frequency response of a micro-machined accelerometer. The indirect coupling between the physical stimulation of the device and its response renders such characterizations somewhat unreliable in many cases. Moreover, such characterizations are often carried out at a convenient location under one set of ambient conditions, such as, for example, the sensitive axis oriented normal to a one-g gravitational field. Unfortunately, the effects of the force normal to the sensitive axis are usually unknown. Such characterizations are assumed to be invariant for other ambient conditions (e.g., a zero-g gravitational field). Such assumptions are dictated by the inconvenience or impossibility of re-characterizing the transducer at a remote location where the other ambient conditions exist.
A further problem is that a micro-machined transducer has such a fine sensitivity that it is highly vulnerable to external or ambient disturbances or forces which tend to mask any smaller parameter whose measurement is sought.